1. Field of the Invention
The present invention relates to an optical fiber which can be suitably used as an optical transmission path and a dispersion compensator.
2. Description of the Related Art
FIG. 14 is a cross-sectional view showing a central portion of an optical fiber including microstructures which has been known conventionally. This optical fiber has a structure in which silica glass 61 constitutes a main medium and a large number of voids (vacant holes) 62 are disposed in a cross section thereof. A central portion in cross section having no voids 62 constitutes a core region 63 and a portion surrounding the core region 63 and including a large number of voids 62 constitutes a cladding region 64.
The principle of light confinement of the optical fiber including such microstructures is explained qualitatively using a concept called effective refractive indices (for example, T. A. Birks et al. Optics Letters Vol. 22 p.961 (1997)). Due to the presence of the microstructures, in a strict sense, the refractive index should show a complicate distribution in the cladding region 64. However, on the assumption that the optical waveguiding characteristics can be approximated by replacing the inside of the cladding region with a homogeneous medium, the refractive index of the homogeneous medium is called the effective refractive index. The effective refractive index neff satisfies a following equation.                               1                      (                                                            f                  1                                                  n                  1                  2                                            +                                                f                  2                                                  n                  2                  2                                                      )                          ≤                  n          eff          2                ≤                                            f              1                        ⁢                          n              1              2                                +                                    f              2                        ⁢                          n              2              2                                                          (        1        )            
where, n is the refractive index and f is the volume fraction. Further, a suffix 1 represents a main medium (silica glass) and a suffix 2 represents a sub medium (air). With respect to the volume refraction, f1+f2=1 is held. Usually, since n1 greater than n2, the both side members in the equation (1) become smaller corresponding to the increase of f2. Accordingly, the effective refractive index of the cladding region 64 including a large number of voids 62 becomes smaller than the effective refractive index of the core region 63 so that the light confinement is realized in the same manner as the usual optical fiber.
Further, a microstructured optical fiber having a greater negative dispersion than the optical fiber shown in FIG. 14 is disclosed in U.S. Pat. No. 5,802,236. As shown in FIG. 15, in this optical fiber, the cladding region is constituted by an inner cladding region and an outer cladding region and by making the void diameter in the inner cladding region greater than the void diameter in the outer cladding region, the effective refractive index of the inner cladding region is made smaller than the outer cladding region.
The previously-mentioned model assumed to define the effective refractive index is considered to be reasonable as long as the optical wavelength is sufficiently long compared to the scale of the microstructure. However, as the optical wavelength becomes shorter, the light is locally concentrated at portions having the high refractive index and hence, it is considered that the assumption that the structure having a non-uniform refractive index distribution can be replaced by a homogeneous medium will lose the validity. As a result, it should be noted that specification of the structure based on effective refractive index is inevitably ambiguous.
In the conventional microstructured optical fiber in which the void diameters in cross section are not uniform, it is difficult to securely realize desired characteristics. This is because, although the void diameters are changed in response to the glass surface tension and/or the internal stress at the time of fiber drawing, the amount of the change depends on the void diameters. For example, when the void diameters are small, the surface tension strongly acts and hence, the contraction is liable to occur compared with the case in which the void diameters are large. As a result, it is difficult to perform the fiber drawing such that each of the voids having different diameters is formed in the fiber with the desired diameters.
The present invention has been made in view of the above circumstances to provide an optical fiber which has sub-medium regions and is capable of securely realizing desired characteristics even when cross sectional areas of the sub-medium regions are changed at the time of fiber drawing.
To solve the problems, according to the present invention, in an optical fiber composed of a core and a cladding surrounding the core, where a given cross section of the cladding includes a plurality of regions made of sub mediums having refractive indices different from the refractive index of the main medium constituting this cladding, wherein the regions made of the sub mediums are arranged in one given or a plurality of given circular annular regions and the centers of the regions made of the sub mediums in each of the circular annular regions are arranged on the same circumference centered at the center of the core and having a diameter predetermined for each of the circular annular regions.
Here, the main medium is a material which can constitute an optical fiber by itself and the regions of the main medium are connected together. On the other hand, the sub mediums may be materials which cannot constitute the optical fiber by themselves and are scattered in a plurality of regions in the optical fiber. A typical main medium is silica glass and a typical sub medium is air or an inert gas.
According to the finding of the inventors of the present inventions, structures containing regions of sub mediums can be specified with less ambiguity using mean refractive index which is expressed by a following equation. Here, assuming that a region can be divided into M sub regions and each of them is formed by a homogeneous medium, the mean refractive index navg of the region can be expressed by the following equation (2).                               n          avg                =                                            ∑                              i                =                1                            M                        ⁢                                          f                i                            ⁢                              n                i                2                                                                        (        2        )            
That is, the mean refractive index navg is the RMS (Root Mean Square) average of the refractive indices of respective mediums weighted by the volume fraction of each medium. Here, ni is the refractive index of the i-th medium and fi is its volume fraction and a following equation holds.                                           ∑                          i              =              1                        M                    ⁢                      f            i                          =        1                            (        3        )            
Accordingly, provided that the regions are determined, the mean refractive index is unambiguously determined. In other words, this implies that the value of the mean refractive index depends on the determination of the regions. According to the optical fiber of the present invention, the sub-medium regions are arranged such that they are positioned in the inside of the given circular annular region and their centers are positioned on given circumferences centered at the center of the core. Due to such a constitution, it becomes easy to design the mean refractive index in each of the circular annular regions so that the optical fiber possesses the desired optical characteristics.
The sub-medium regions can be arranged on the circumferences centered at the center of the core so that the arrangement has the four-fold rotational symmetry with respect to the center. Such a constitution is preferable to decrease the mode birefringence and the polarization mode dispersion.
At least one of cross-sectional areas and the refractive indices of these regions made of the sub mediums may change along the fiber axis. Due to such a constitution, the mean refractive index of the circular annular region including the sub mediums can be changed along the fiber axis. Accordingly, the optical fiber whose optical characteristics are changed along its axis can be easily realized.
Further, it is preferable that sections where the cladding does not contain the sub mediums are spaced along the fiber axis. Thus, by providing the sections having no sub-medium regions in cross section, it becomes possible to cleave the optical fiber and splice it to another optical fiber by fusion without excess transmission loss at the splice due to deformation of the structure and contamination of the sub-medium regions.
Further, it is preferable that the chromatic dispersion at a given wavelength is changed along the fiber axis. Due to such a provision, a chromatic dispersion managed fiber composed of plural fiber sections which differ in the chromatic dispersion characteristics can be realized and the optical characteristics whose realization is difficult or impossible with an optical fiber composed of a single kind of fiber section can be realized. For example, the characteristics that absolute value of the total chromatic dispersion is small over a broad wavelength range and the characteristics that the absolute value of the local chromatic dispersion is large and the absolute value of the total chromatic dispersion is small can be realized.
Further, an optical fiber includes at least a section of the first kind where the chromatic dispersion at a given wavelength is positive and a section of the second kind where the chromatic dispersion at the same wavelength is negative can be realized. This optical fiber is suitable as a transmission path for an optical communication of a large capacity. This is because the deterioration of the transmission quality due to the non-linear optical phenomena generated among optical signals having different wavelengths can be made small.
It is preferable that the chromatic dispersion at the given wavelength band is larger than +1 ps/nm/km in the section of the first kind and is smaller than xe2x88x921 ps/nm/km in the section of the second kind, and the total length of the fiber sections whose absolute value of the chromatic dispersion becomes below 1 ps/nm/km at the wavelength is below {fraction (1/10)} of the full length of the optical fiber. By properly setting the lengths and the chromatic dispersion values of respective fiber sections, the expansion of the optical pulses derived from the total chromatic dispersion can be made small. As a result, an optical fiber which exhibits the least deterioration of the transmission quality derived from the nonlinear optical phenomena among optical signals having different wavelengths and the least expansion of the optical pulses derived from the total chromatic dispersion and hence is suitable as a transmission path for an optical communication of a large capacity can be realized.
It is preferable that the chromatic dispersion slopes at the given wavelength band in the section of the first kind and the section of the second kind have opposite signs. By setting the chromatic dispersion slopes in this manner, an optical fiber having an absolute value of a total chromatic dispersion which is smaller than a given value in a given wavelength band can be obtained. As a result, the absolute value of the total chromatic dispersion can be made small over a broad wavelength range so that the transmittable capacity of optical signals can be increased.
Further, it is preferable that the chromatic dispersion slope at a given wavelength is negative in the section of the first kind and is positive in the section of the second kind. Due to such a constitution, an optical fiber in which a locally-zero-dispersion wavelength which makes the local chromatic dispersion zero in fiber sections having not less than a given length is present in the longer wavelength side of the operating wavelength band can be realized. The wavelength band in the vicinity of the locally-zero-dispersion wavelength cannot be used for the wavelength division multiplexing transmission due to the deterioration of the transmission quality dei to the nonlinear optical phenomena generated among optical signals having different wavelengths. However, the wavelength band remote from the locally-zero-dispersion wavelength can be used by using a suitable dispersion compensator. According to the present invention, an optical fiber having no locally-zero-dispersion wavelength in the short-wavelength side of the operating wavelength band which cannot be realized by the prior art can be realized and hence, the expansion of the operating wavelength band toward the short wavelength side can be realized.
It is preferable that the structural densities of the sub-medium regions is different between at least the two circular annular regions. By making the structural densities of the sub-medium regions different between the circular annular regions, the mean refractive indices can be easily made different between the circular annular regions. Here, the structural density means the number of sub-medium regions per unit cross-sectional area of the fiber.
Further, it is preferable that the cross-sectional areas of respective regions made of sub mediums are substantially uniform in the transverse cross section of each of the circular annular region. Due to such a constitution, the realization of desired optical characteristics can be facilitated. Usually, the area fraction of the sub-medium, that is, the proportion of the area occupied by the sub medium in a given region, is changed at the time of fiber drawing. Although this change depends on the areas of respective sub-medium regions, the change is substantially uniform in the inside of the circular annular region. Accordingly, by setting the areas of the sub-medium regions in the inside of the circular annular region uniform, the change of the area fraction of the sub medium becomes substantially uniform. As a result, by adjusting the fiber drawing conditions such that one sub-medium region has a desired area fraction, it becomes possible to realize the desired values of area fractions for the area fractions of other sub-medium regions in the inside of the circular annular region.
Further, it is preferable that the cross-sectional areas of the regions made of the sub mediums are all substantially uniform in the transverse cross section of the fiber. Due to such a constitution, by merely adjusting the fabrication condition so that one of the sub-medium regions have the desired value of the area fraction, the mean refractive index of each circular annular region can be adjusted to a desired value at the same time so that the optical fibers can be easily and securely fabricated with the desired characteristics.
The optical fiber according to the present invention may include at least two cladding regions, and structural densities of the region made of the sub mediums are different between the two cladding regions.
In the optical fiber of the present invention, the mean refractive index distribution in the cross section of the optical fiber may be changed by controlling the structural densities of the sub medium regions. That is, compared with the prior art, even when the numbers of the sub-medium regions are short, a desired mean refractive index distribution can be realized. Although the cross-sectional areas of the sub-medium regions are liable to be changed during fiber drawing, their structural densities are hardly changed. Accordingly, the control of the fiber drawing becomes easy and the stable fabrication of products becomes possible.
For example, by increasing (decreasing) the structural density of regions made of sub mediums having the lower (higher) refractive index than the main medium, the mean refractive index can be decreased and by performing the inverse change of the structural density, the mean refractive index can be increased whereby the range in which mean refractive index can be realized can be broadened.
Here, the maximum value of the cross-sectional areas of the regions made of the sub mediums in the cross section of the fiber may be smaller than 10 times of the minimum value of them. This is because, in the case that the cross-sectional area of sub-medium regions is changed, since the manner of change of the area fraction of the sub-medium at the time of fiber drawing depends on the cross sectional area of sub-medium regions, when the maximum cross-sectional area and the minimum cross-sectional area differ greatly, it is difficult to realize a desired cross-sectional area of sub-medium regions and accordingly the desired optical characteristics.
It is further preferable that the cross-sectional areas of the regions made of the sub mediums are all substantially uniform in the cross section of the fiber. By making the cross-sectional areas of the sub-medium regions in the transverse cross section of the fiber all substantially uniform, the areas of the sub-medium regions can be realized as desired if only one of them is realized as desired. Accordingly, the optical fiber can be easily and securely fabricated with desired optical characteristics.